set ns [new Simulator]
$ns node-config -Routing protoname  
set nf [open out.nam w]    
$ns namtrace-all $nf      
set nd [open out.tr w]      
$ns trace-all $nd            
	proc finish {} {
          global ns nf  nd
          $ns flush-trace
          close $nf      
          close $nd      
          exec nam out.nam &
          exit 0
	}

for {set i 0} {$i < 7} {incr i} {set n($i) [$ns node] }

for {set i 0} {$i < 7} {incr i} {
	$ns duplex-link $n($i) $n([expr ($i+1)%7]) 1Mb 10ms DropTail
}

set udp0 [new Agent/UDP]  
$ns attach-agent $n(0) $udp0
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500    
$cbr0 set interval_ 0.005     
$cbr0 attach-agent $udp0
set null0 [new Agent/Null]
$ns attach-agent $n(3) $null0
$ns connect $udp0 $null0
$ns at 0.5 "$cbr0 start"
$ns rtmodel-at 1.0 down $n(1) $n(2)
$ns rtmodel-at 2.0 up $n(1) $n(2)  
$ns at 4.5 "$cbr0 stop"
$ns at 5.0 "finish"
$ns run